.TH std::ftell 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::ftell \- std::ftell

.SH Synopsis
   Defined in header <cstdio>
   long ftell( std::FILE* stream );

   Returns the current value of the file position indicator for the file stream stream.

   If the stream is open in binary mode, the value obtained by this function is the
   number of bytes from the beginning of the file.

   If the stream is open in text mode, the value returned by this function is
   unspecified and is only meaningful as the input to std::fseek.

.SH Parameters

   stream - file stream to examine

.SH Return value

   File position indicator on success or -1L if failure occurs. Also sets errno on
   failure.

.SH Example

   Demonstrates std::ftell() with error checking. Writes then reads a few
   floating-point (FP) values to/from a file.


// Run this code

 #include <cstdio>
 #include <cstdlib>
 #include <iostream>

 // If the condition is not met then exit the program with error message.
 void check(bool condition, const char* func, int line)
 {
     if (condition)
         return;
     std::perror(func);
     std::cerr << func << " failed in file " << __FILE__ << " at line # " << line - 1
               << '\\n';
     std::exit(EXIT_FAILURE);
 }

 int main()
 {
     // Prepare an array of FP values.
     constexpr int SIZE {5};
     double A[SIZE] = {1.1, 2.2, 3.3, 4.4, 5.5};

     // Write array to a file.
     const char* fname = "/tmp/test.bin";
     FILE* file = std::fopen(fname, "wb");
     check(file != NULL, "fopen()", __LINE__);

     const int write_count = std::fwrite(A, sizeof(double), SIZE, file);
     check(write_count == SIZE, "fwrite()", __LINE__);

     std::fclose(file);

     // Read the FP values into array B.
     double B[SIZE];
     file = std::fopen(fname, "rb");
     check(file != NULL, "fopen()", __LINE__);

     long pos = std::ftell(file); // position indicator at start of file
     check(pos != -1L, "ftell()", __LINE__);
     std::cout << "pos: " << pos << '\\n';

     const int read_count = std::fread(B, sizeof(double), 1, file); // read one FP value
     check(read_count == 1, "fread()", __LINE__);

     pos = std::ftell(file); // position indicator after reading one FP value
     check(pos != -1L, "ftell()", __LINE__);
     std::cout << "pos: " << pos << '\\n';
     std::cout << "B[0]: " << B[0] << '\\n'; // print one FP value

     return EXIT_SUCCESS;
 }

.SH Possible output:

 pos: 0
 pos: 8
 B[0]: 1.1

.SH See also

   fgetpos gets the file position indicator
           \fI(function)\fP
   fseek   moves the file position indicator to a specific location in a file
           \fI(function)\fP
   fsetpos moves the file position indicator to a specific location in a file
           \fI(function)\fP
   tellg   returns the input position indicator
           \fI(public member function of std::basic_istream<CharT,Traits>)\fP
   tellp   returns the output position indicator
           \fI(public member function of std::basic_ostream<CharT,Traits>)\fP
   C documentation for
   ftell
